1. Field of the Invention
This invention relates to a signal band expanding method and apparatus and signal synthesis method and apparatus in which speech signals of a narrow frequency range, transmitted by communication or broadcasting or stored in a medium, or parameters making up the signals, are transmitted over a transmission path or directly recorded on the medium, so as to be used on the reception or reproducing side for estimating the broad-band speech signals on the receiving or reproducing side, and which may be used with advantage especially in a portable telephone terminal having the band expanding function.
2. Description of the Related Art
The bandwidth of the telephone network is narrow such as 300 to 3400 such that limitations are imposed on the frequency band of speech signals sent over the telephone network. Therefore, the sound quality of the conventional analog telephone network cannot be said to be optimum. The digital portable telephone also is not satisfactory in sound quality.
However, since the standard of the transmission path is fixed, it is difficult to enlarge its bandwidth. Thus, a variety of systems are now proposed for predicting signal components outside the band on the receiving side to generate broad-band signals.
In particular, in systems exploiting the vector sum excited linear prediction (VSELP) coding or pitch synchronization innovationxe2x80x94code excited linear prediction (PSI-CELP), which are the speech codec system for car/portable telephone in Japan, attention is directed to LPC synthesis, both the linear prediction coefficients xcex1 and the excitation source are enlarged in the frequency range and LPC synthesis is made by xcex1 and the excitation source of the broad bandwidth.
However, the broad band-speech, thus obtained, suffers from distortion. Therefore, in the frequency component contained in the original speech, the original speech is naturally of higher quality, and hence these components contained in the synthesized broad-band speech are filtered off and summed to the original speech.
For combatting the overflow in the digital signal processing, there are known methods of clipping the digital signal to a maximum value or of adjusting the gain of the entire signal to prevent signal overflow.
However, if overflow occurs in the process of addition of main signals and sub-signals, and it is desired not to change the main signal even if the sub-signal is eliminated in its entirety, these overflow combatting measures are not optimum.
There is also known a technique in which the speech of the vector sum excited linear prediction (VSELP) coding and pitch synchronization innovationxe2x80x94code excited linear prediction (PSI-CELP) coding system, as the speech codec of the car/portable telephone in the personal digital cellular (PDC) system, having the frequency bandwidth of 300 to 3400 Hz, is enlarged in bandwidth to approximately 300 to 6000 Hz by estimating the signal components outside the band on the receiving side. In this technique, the signals outside the transmission bandwidth is synthesized and summed to the narrow band signals corresponding to the original speech signals.
Among transmitted narrrow band parameters, there are a linear prediction coefficient xcex1, a reflection coefficient k and a line spectrum pair (LSP). These represent the speech spectrum envelope, with the number of orders of the coefficients corresponding to peaks of the spectrum. In the PDC system, up to the tenth order coefficients are transmitted, in consideration that the number of formants in the human voice up to approximately 3400 Hz is on the order of five.
One of a wide variety of possible prediction methods for the wide range parameter representing the wide band formant exploits vector quantization. In this method, a number of vectors corresponding to the number of orders of the broad band parameters are prepared by previous learning and, on inputting of the narrow band parameter, a suitable broad band vector is selected from these parameters as the broad band parameter.
It has now been found that, in the broad band speech, thus synthesized, there exists a marked difference in personal appreciation of the sound quality and hence it is preferred not to fix the gain of the high range component synthesized by prediction. Similarly, the high range component not less than 6 kHz, for which the general preference is moderate suppression, also is preferably not fixed.
It is therefore an object of the first subject-matter of the present invention to provide a bandwidth expanding method and apparatus in which frequency characteristics of high-frequency components can be adjusted to the liking of users.
On the other hand, in the above-described bandwidth expansion technique, overflow by addition is eventually produced. However, the main signal needs to be the original signal at any rate, while the component outside the transmission band is not needed at the cost of generation of extraneous sound ascribable to overflow.
It is therefore not desirable to clip the signal at the maximum value to produce extraneous sound or to adjust the entire signal to produce perceptible power variations, and hence an alternative overflow combatting technique is desired.
It is therefore an object of the second subject-matter of the present invention to provide a signal processing method and apparatus for suppressing overflow by adjusting only the signals of the subsidiary system.
It is also an object of the second subject-matter of the present invention to provide a bandwidth expanding method and apparatus in which it is possible to suppress overflow and to expand the bandwidth without changing the low range signals to improve spontaneity in hearing.
In addition, in estimating and synthesizing the broad-band speech from the narrow band parameters, transmitted as described above, the number of formants naturally is larger than that for the narrow bands, that is five.
The increased number of formants is not meritorious since comparison is then made of finer components of the spectrum envelope to depart from the inherent intention of roughly estimating the broad-band spectrum envelope.
It is therefore an object of the third subject-matter of the present invention to provide a speech band expanding method and apparatus and speech synthesis method and apparatus in which the number of broad-band formants can be diminished, importance can be attached to the rough structure of the spectrum, the broad-band speech can be improved in quality and in which the processing volume required in the memory capacity and codebook searching can be saved.
In connection with the first subject-matter, the present invention provides a bandwidth expanding method for expanding a bandwidth by estimating, from narrow-band signals or from parameters allowing for synthesizing the narrow-band signals, outside-band components, and by adding the outside-band components to the narrow-band signals, wherein frequency characteristics of the outside-band components are first adjusted by pre-set alterable parameter values and subsequently the outside-band components are added to the narrow-band signals.
In connection with the first subject-matter, the present invention provides a bandwidth expanding apparatus for expanding the bandwidth by estimating, from narrow-band signals or from parameters allowing for synthesizing the narrow-band signals, outside-band components, and by adding the outside-band components to the narrow-band signals, wherein the apparatus includes frequency characteristics adjustment means for adjusting frequency characteristics of the outside-band components by pre-set alterable parameter values, and addition means for adding the outside-band components, the frequency characteristics of which have been adjusted by the frequency characteristics adjustment means, to the narrow-band signals.
In connection with the first subject-matter, the present invention provides a bandwidth expanding apparatus for expanding the bandwidth by estimating, from narrow-band signals or from parameters allowing for synthesizing the narrow-band signals, outside-band components, and by adding the outside-band components to the narrow-band signals, including addition means for adding the outside-band components to the narrow-band signals, and frequency characteristics adjustment means for adjusting the frequency characteristics of the outside-band components for adjusting frequency characteristics of the outside-band components of an addition output of the addition means by pre-set alterable parameters.
In connection with the second subject-matter, the present invention provides a signal processing method for adding signals of a main system to signals of a subsidiary system, wherein, before adding the signals of the subsidiary system to the signals of the main system, the gain of a given sample of the signals of the sub-system and the gain of samples following the given sample are adjusted based on the presence or absence of the overflow that can be determined from an amount of addition.
In connection with the second subject-matter, the present invention provides a signal processing apparatus for adding signals of a main system to signals of a subsidiary system, including addition means for summing the signals of the subsidiary system to signals of the main system, overflow detection means for detecting the presence or absence of overflow that can be verified from an amount of addition from the addition means, gain adjustment means for adjusting the gain for the given sample and the following samples of the signals of the subsidiary system based on the detected results from the overflow detection means, and multiplication means for multiplying the given and following samples of the signals of the subsidiary system by an adjustment gain from the gain adjustment means.
In connection with the second subject-matter, the present invention provides a bandwidth expanding method for expanding the bandwidth by estimating, from narrow-band signals or from parameters allowing for synthesizing the narrow-band signals, outside-band components, and by adding the outside-band components to the narrow-band signals, wherein, before adding the outside-band components to the narrow-band signals, the gain of the outside-band components is adjusted based on the presence or absence of overflow that can be determined from an amount of addition.
In connection with the second subject-matter, the present invention provides a bandwidth expanding apparatus for expanding the bandwidth by estimating, from narrow-band signals or from parameters allowing for synthesizing the narrow-band signals, outside-band components, and by adding the outside-band components to the narrow-band signals, wherein the apparatus includes addition means for summing the outside-band components to the narrow-band signals, overflow detection means for detecting the presence or absence of overflow that can be verified from an amount of addition from the addition means, gain adjustment means for adjusting the gain for the given sample and the following samples of the outside-band components based on detected results from the overflow detection means and multiplication means for multiplying the given and following samples of the outside-band components by an adjustment gain from the gain adjustment means.
In connection with the third subject-matter, the present invention provides a speech bandwidth expanding method including a parameter extraction step for producing from input narrow band signals aparameter that allows representation of the narrow-range formant, a parameter prediction step for predicting a parameter that allows representation of a number of broad band formants not larger than the number of the produced narrow-band formants from the input narrow band speech signal, and a synthesis step for synthesizing the broad-band speech from a parameter that allows for representation of the produced broad band formants.
In connection with the third subject-matter, the present invention provides a speech bandwidth expanding apparatus including parameter extraction means for producing from input narrow band signals a parameter that allows representation of the narrow-range formant, parameter prediction means for predicting a parameter that allows representation of a number of broad band formants not larger than the number of the produced narrow-band formants, and synthesis means for synthesizing the broad-band speech from a parameter that allows for representation of the produced broad band formants.
In connection with the third subject-matter, the present invention provides a speech synthesis method including a first parameter extraction step for predicting parameters that allow for representation of a number of the broad band formants not larger than the number of narrow band narrow band formants from narrow band parameters representing the input narrow band speech and which allow for representation of the input narrow band speech, a parameter extraction step for producing parameters that allow representation of the narrow-range formant information from the input narrow band speech, a second parameter prediction step for predicting a parameter that allows representation of a number of broad band formants not larger than the number of the produced narrow-band formants, and a synthesis step for synthesizing the broad-band speech from a parameter that allows for representation of the produced broad band formants.
In connection with the third subject-matter, the present invention provides a speech synthesis apparatus including first parameter extraction means for predicting parameters that allow for representation of a number of the broad band formants not larger than the number of narrow band narrow band formants from narrow band parameters representing the input narrow band speech and which allow for representation of the input narrow band speech, parameter extraction means for producing parameters that allow representation of the narrow-range formant information from the input narrow band speech, second parameter prediction means for predicting a parameter that allows representation of a number of broad band formants not larger than the number of the produced narrow-band formants, and synthesis means for synthesizing the broad-band speech from a parameter that allows for representation of the produced broad band formants.
With the bandwidth enlarging method and apparatus according to the first subject-matter of the present invention, the frequency characteristics of high frequency components, such as gain, is rendered alterable to provide the broad-band speech suited to the liking of the user.
With the signal processing method and apparatus according to the second subject-matter of the present invention, it is possible to make the best use of the characteristics of the main system signals because overflow can be prevented from occurring by adjusting only the signals of the subsidiary system.
With the bandwidth enlarging method and apparatus according to the second subject-matter of the present invention, it is possible to prevent overflow without changing the low range side signals as main system signals and to enlarge the bandwidth to improve spontaneity in hearing.
With the speech band enlarging method and apparatus and the speech synthesis method and apparatus according to the third subject-matter of the present invention, in which the broad-band speech is predicted and synthesized from the narrow band speech or from the narrow band parameters, it is possible to diminish the number of formants of the synthesized broad-band speech to attach more importance to the rough spectral structure to improve the quality of the produced broad-band speech as well as to save the memory capacity and the processing volume required in codebook search.